No. 6 or No. 8 blasting caps have been reliably used for many years as initiators for nitroglycerin based dynamite. However, with comparatively insensitive explosives, a No. 6 or No. 8 cap does not initiate the explosive. Such is the case with the most common industrial explosive, ANFO (ammonium nitrate/fuel oil). Other insensitive explosives include many of the relatively new gels and emulsions. The need for a reliable method of detonating comparatively insensitive explosives was partially met with the advent of the booster cap, which is used to enhance the output of the blasting cap and is initiated by the blasting cap.
The explosives industry has sought a booster cap which would be impervious to water desensitization and dudding, and shippable under U.S. Department of Transportation (DOT) Explosives A, Type 8 (miscellaneous) classification, thereby greatly reducing cost and increasing availability of such booster cap assemblies. In the past, booster cap assemblies possessing imperviousness to water desensitization have not possessed good shippability characteristics because in the past booster caps with good resistance to water desensitization were also inherently hazardous in the presence of heat or flame, necessitating shipment under DOT Explosives A, Type 7 (blasting cap) regulations. Similarly, booster caps with good shippability characteristics had inherently poor water desensitization characteristics. Thus, in the past these two desired characteristics have not been compatible. Previous attempts to combine both of these characteristics in a booster cap have resulted in caps containing neither excellent resistance to water desensitization nor good shippability characteristics. For example, by inserting a blasting cap into the booster, a small degree of water tightness was provided in Graham, U.S. Pat. No. Re. 28,228.
It has also been known to provide a small vent hole in an otherwise enclosed booster cap to allow outgassing and venting when subjected to fire. However, such booster caps had poor water resistance. Thus, both of the aforementioned teachings were subject to either complete or partial desensitization through water invasion. A need exists for a blasting cap booster that is water-tight, but when subjected to fire or excessive heat will not explode.